diff --git a/src/playerprogress.ts b/src/playerprogress.ts
index 88434d3..a73f4cf 100644
--- a/src/playerprogress.ts
+++ b/src/playerprogress.ts
@@ -1,6 +1,12 @@
 import { ALL_STATS, Skill, Stat, SuccessorOption, Wish } from "./datatypes.ts";
 import { getSkills } from "./skills.ts";
 import { getThralls, ItemStage, LifeStage, Thrall } from "./thralls.ts";
+import { SaveFileV1, mustBeSaveFileV1 } from "./saveformat.ts";
+
+interface NewRoundConfig {
+  asSuccessor: SuccessorOption,
+  withWish: Wish | null,
+}
 
 export class PlayerProgress {
   #name: string;
@@ -20,25 +26,61 @@ export class PlayerProgress {
   #thrallsObtainedItem: number[];
   #thrallsDeliveredItem: number[];
 
-  constructor(asSuccessor: SuccessorOption, withWish: Wish | null) {
-    this.#name = asSuccessor.name;
-    this.#thrallTemplate = asSuccessor.template.id;
-    this.#nImprovements = asSuccessor.nImprovements;
-    this.#stats = { ...asSuccessor.stats };
-    this.#talents = { ...asSuccessor.talents };
-    this.#isInPenance = asSuccessor.inPenance;
-    this.#wish = withWish;
-    this.#exp = 0;
-    this.#blood = 0;
-    this.#itemsPurloined = 0;
-    this.#skillsLearned = [];
-    this.#untrimmedSkillsAvailable = [];
-    this.#thrallsUnlocked = [];
-    this.#thrallDamage = {};
-    this.#thrallsObtainedItem = [];
-    this.#thrallsDeliveredItem = [];
+  constructor(args: NewRoundConfig | SaveFileV1) {
+    if ("asSuccesor" in args) {
+    //asSuccessor: SuccessorOption, withWish: Wish | null) {
+      const config = args as NewRoundConfig;
+      const asSuccessor = config.asSuccessor;
+      this.#name = asSuccessor.name;
+      this.#thrallTemplate = asSuccessor.template.id;
+      this.#nImprovements = asSuccessor.nImprovements;
+      this.#stats = { ...asSuccessor.stats };
+      this.#talents = { ...asSuccessor.talents };
+      this.#isInPenance = asSuccessor.inPenance;
+      this.#wish = config.withWish;
+      this.#exp = 0;
+      this.#blood = 0;
+      this.#itemsPurloined = 0;
+      this.#skillsLearned = [];
+      this.#untrimmedSkillsAvailable = [];
+      this.#thrallsUnlocked = [];
+      this.#thrallDamage = {};
+      this.#thrallsObtainedItem = [];
+      this.#thrallsDeliveredItem = [];
 
-    this.refill();
+      this.refill();
+    } else {
+      const file = mustBeSaveFileV1(args);
+      this.#name = file.name;
+      this.#thrallTemplate = file.thrallTemplateId;
+      this.#nImprovements = file.nImprovements;
+      this.#stats = {
+        AGI: file.stats.agi,
+        INT: file.stats.int,
+        CHA: file.stats.cha,
+        PSI: file.stats.psi,
+      }
+      this.#talents = {
+        AGI: file.talents.agi,
+        INT: file.talents.int,
+        CHA: file.talents.cha,
+        PSI: file.talents.psi,
+      }
+      this.#isInPenance = file.isInPenance,
+      this.#wish = file.wishId >= 0 ? {id: file.wishId} : null;
+      this.#exp = file.exp;
+      this.#blood = file.blood;
+      this.#itemsPurloined = file.itemsPurloined;
+      this.#skillsLearned = file.skillsLearned;
+      this.#untrimmedSkillsAvailable = file.untrimmedSkillsAvailableIds.map((id) => {return {id: id}});
+      this.#thrallsUnlocked = file.thrallsUnlocked;
+      this.#thrallDamage = {};
+      for(let i = 0; i < file.thrallDamage.length; ++i) {
+        this.#thrallDamage[i] = file.thrallDamage[i];
+      }
+      this.#thrallsObtainedItem = file.thrallsObtainedItem;
+      this.#thrallsDeliveredItem = file.thrallsDeliveredItem;
+    }
   }
 
   applyEndOfTurn() {
@@ -332,7 +374,7 @@ export function initPlayerProgress(
   asSuccessor: SuccessorOption,
   withWish: Wish | null,
 ) {
-  active = new PlayerProgress(asSuccessor, withWish);
+  active = new PlayerProgress({asSuccessor:asSuccessor, withWish:withWish});
 }
 
 export function getPlayerProgress(): PlayerProgress {
diff --git a/src/save.ts b/src/save.ts
index e4900ad..68d7db7 100644
--- a/src/save.ts
+++ b/src/save.ts
@@ -1,44 +1,14 @@
 import { getPlayerProgress } from "./playerprogress"
 import { getStateManager } from "./statemanager";
 import { getThralls } from "./thralls";
+import {SaveFileV1, StatCounterV1} from "./saveformat";
 
 export interface SaveFile {
     version: string;
     revision: number;
 }
 
-export interface StatCounterV1{
-    agi: number;
-    int: number;
-    cha: number;
-    psi: number;
-}
-
-export interface SaveFileV1 {
-    version: "fledgling_save_v1";
-    revision: number;
-
-    turn: number;
-
-    name: string;
-    thrallTemplateId: number;
-    nImprovements: number;
-    stats: StatCounterV1;
-    talents: StatCounterV1;
-    isInPenance: boolean;
-    wishId: number;  // negative: Wish is absent
-    exp: number;
-    blood: number;
-    itemsPurloined: number;
-    skillsLearned: number[];
-    untrimmedSkillsAvailableIds: number[];
-    thrallsUnlocked: number[];
-    thrallDamage: number[];  // 0: thrall is absent or undamaged
-    thrallsObtainedItem: number[];
-    thrallsDeliveredItem: number[];
-}
-
-export type SaveSlot = "FLEDGLING_SLOT_1" | "FLEDGLING_SLOT_2";
+type SaveSlot = "FLEDGLING_SLOT_1" | "FLEDGLING_SLOT_2";
 
 /// Checks whether obj is a valid save file, as far as we can tell, and returns
 /// it unchanged if it is, or throws an error if it's not valid.
diff --git a/src/saveformat.ts b/src/saveformat.ts
new file mode 100644
index 0000000..c141c30
--- /dev/null
+++ b/src/saveformat.ts
@@ -0,0 +1,180 @@
+export interface StatCounterV1{
+    agi: number;
+    int: number;
+    cha: number;
+    psi: number;
+}
+
+export interface SaveFileV1 {
+    version: "fledgling_save_v1";
+    revision: number;
+
+    turn: number;
+
+    name: string;
+    thrallTemplateId: number;
+    nImprovements: number;
+    stats: StatCounterV1;
+    talents: StatCounterV1;
+    isInPenance: boolean;
+    wishId: number;  // negative: Wish is absent
+    exp: number;
+    blood: number;
+    itemsPurloined: number;
+    skillsLearned: number[];
+    untrimmedSkillsAvailableIds: number[];
+    thrallsUnlocked: number[];
+    thrallDamage: number[];  // 0: thrall is absent or undamaged
+    thrallsObtainedItem: number[];
+    thrallsDeliveredItem: number[];
+}
+
+/// Checks whether obj is a valid save file, as far as we can tell, and returns
+/// it unchanged if it is, or throws an error if it's not valid.
+export function mustBeSaveFileV1(obj: unknown): SaveFileV1 {
+    if (obj === undefined || obj === null) {
+        throw new Error("nonexistent");
+    }
+    if (typeof(obj) !== "object") {
+        throw new Error(`not an object; was ${typeof(obj)}`);
+    }
+
+    if (!("version" in obj)) {
+        throw new Error("no magic number");
+    }
+    if (obj.version !== "fledgling_save_v1") {
+        throw new Error(`bad magic number: ${obj.version}`);
+    }
+
+    return {
+        version: "fledgling_save_v1",
+        revision: mustGetNumber(obj, "revision"),
+        turn: mustGetNumber(obj, "turn"),
+        name: mustGetString(obj, "name"),
+        thrallTemplateId: mustGetNumber(obj, "thrallTemplateId"),
+        nImprovements: mustGetNumber(obj, "nImprovements"),
+        stats: mustGetStatCounterV1(obj, "stats"),
+        talents: mustGetStatCounterV1(obj, "talents"),
+        isInPenance: mustGetBoolean(obj, "isInPenance"),
+        wishId: mustGetNumber(obj, "wishId"),
+        exp: mustGetNumber(obj, "exp"),
+        blood: mustGetNumber(obj, "blood"),
+        itemsPurloined: mustGetNumber(obj, "itemsPurloined"),
+        skillsLearned: mustGetNumberArray(obj, "skillsLearned"),
+        untrimmedSkillsAvailableIds: mustGetNumberArray(obj, "untrimmedSkillsAvailableIds"),
+        thrallsUnlocked: mustGetNumberArray(obj, "thrallsUnlocked"),
+        thrallDamage: mustGetNumberArray(obj, "thrallDamage"),
+        thrallsObtainedItem: mustGetNumberArray(obj, "thrallsObtainedItem"),
+        thrallsDeliveredItem: mustGetNumberArray(obj, "thrallsDeliveredItem"),
+    };
+}
+
+function mustGetNumber(obj: object, key: string) : number {
+    if (obj === null || obj === undefined) {
+        throw new Error("container absent");
+    }
+    if (typeof(obj) !== "object") {
+        throw new Error(`container was not an object; was ${typeof(obj)}`);
+    }
+    if (!(key in obj)) {
+        throw new Error(`missing number: ${key}`);
+    }
+    const dict = obj as {[key:string]:any};
+    const val = dict[key];
+    if (typeof(val) !== "number") {
+        throw new Error(`not a number: ${key}: ${val}`);
+    }
+    return val;
+}
+
+function mustGetString(obj: object, key: string) : string {
+    if (obj === null || obj === undefined) {
+        throw new Error("container absent");
+    }
+    if (typeof(obj) !== "object") {
+        throw new Error(`container was not an object; was ${typeof(obj)}`);
+    }
+    if (!(key in obj)) {
+        throw new Error(`missing number: ${key}`);
+    }
+    const dict = obj as {[key:string]:any};
+    const val = dict[key];
+    if (typeof(val) !== "string") {
+        throw new Error(`not a string: ${key}: ${val}`);
+    }
+    return val;
+}
+
+function mustGetStatCounterV1(obj: object, key: string) : StatCounterV1 {
+    if (obj === null || obj === undefined) {
+        throw new Error("container absent");
+    }
+    if (typeof(obj) !== "object") {
+        throw new Error(`container was not an object; was ${typeof(obj)}`);
+    }
+    if (!(key in obj)) {
+        throw new Error(`missing number: ${key}`);
+    }
+    const dict = obj as {[key:string]:any};
+    const val = dict[key];
+    if (typeof(val) !== "object") {
+        throw new Error(`not an object: ${key}: ${val}`);
+    }
+
+    try {
+        return {
+            agi: mustGetNumber(val, "agi"),
+            int: mustGetNumber(val, "int"),
+            cha: mustGetNumber(val, "cha"),
+            psi: mustGetNumber(val, "psi"),
+        };
+    } catch (e) {
+        let message = "unrecognizable error";
+        if (e instanceof Error) {
+            message = e.message;
+        }
+        throw new Error(`reading ${key}: ${message}`);
+    }
+}
+
+function mustGetBoolean(obj: object, key: string) : boolean {
+    if (obj === null || obj === undefined) {
+        throw new Error("container absent");
+    }
+    if (typeof(obj) !== "object") {
+        throw new Error(`container was not an object; was ${typeof(obj)}`);
+    }
+    if (!(key in obj)) {
+        throw new Error(`missing number: ${key}`);
+    }
+    const dict = obj as {[key:string]:any};
+    const val = dict[key];
+    if (typeof(val) !== "boolean") {
+        throw new Error(`not boolean: ${key}: ${val}`);
+    }
+    return val;
+}
+
+function mustGetNumberArray(obj: object, key: string) : number[] {
+    if (obj === null || obj === undefined) {
+        throw new Error("container absent");
+    }
+    if (typeof(obj) !== "object") {
+        throw new Error(`container was not an object; was ${typeof(obj)}`);
+    }
+    if (!(key in obj)) {
+        throw new Error(`missing number: ${key}`);
+    }
+    const dict = obj as {[key:string]:any};
+    const val = dict[key];
+    if (typeof(val) !== "object") {
+        throw new Error(`not an object: ${key}: ${val}`);
+    }
+
+    for (const x of val) {
+        if (typeof(x) !== "number") {
+            throw new Error(`contained non-number item in ${key}: ${val}`);
+        }
+    }
+    return val;
+}
\ No newline at end of file