补充一下楼上的答案，wiki里面其实有一段剧情介绍，我贴一段原文（https://en.wikipedia.org/wiki/The_Binding_of_Isaac_(video_game)）加翻译。
The Binding of Isaac's plot is very loosely inspired by the biblical story of the same name.[4] Isaac, a child, and his mother live in a small house on a hill, both happily keeping to themselves, with Isaac drawing pictures and playing with his toys, and his mother watching Christian broadcasts on television. Isaac's mother then hears "a voice from above", stating her son is corrupted with sin, and needs to be saved. It asks her to remove all that was evil from Isaac, in an attempt to save him. His mother obliges, taking away his toys, drawings, and even his clothes.
The voice once again speaks to Isaac's mother, stating that Isaac must be cut off from all that is evil in the world. Once again, his mother obliges, and locks Isaac inside his room. Once more, the voice speaks to Isaac's mother. It states she has done well, but it still questions her devotion, and tells her to sacrifice her son. She obliges, grabbing a butcher's knife from the kitchen and walking to Isaac's room. Isaac, watching through a sizable crack in his door, starts to panic. He finds a trapdoor hidden under his rug and jumps in, just before his mother opens his bedroom door. Isaac then puts the paper he was drawing onto his wall, which becomes the title screen.
During the game's loading points, Isaac is shown curled up in a ball, crying. His thoughts are visible, ranging among rejection from his mother and humiliation from his peers to a scenario involving his own death. The game features 13 possible endings, one after each major boss fight.
翻译大意如下：
《以撒的结合》的剧情在一定程度上受到了圣经故事“以撒的结合”的启发。以撒是一个小孩子，和他的母亲住在山上的一座小房子里。母子两人过着快乐守成的生活，以撒喜欢画画和玩玩具，他的母亲则喜欢看基督教电视广播。后来，以撒的母亲听到了“来自天国的声音”，说她的儿子沾染了原罪，需要被救赎。他的母亲听从了教导，拿走了他的玩具、画作——甚至是他的衣服。
然后，以撒的母亲又听到了那个声音，它说以撒必须被隔离起来，远离世上所有邪恶的东西。他的母亲再次听从了教诲，把以撒锁在了他自己的房间里。不久，那声音又对以撒的母亲说话了。它说她做得很好，但她的虔诚仍不能让人信服，它告诉她应该用她的儿子来献祭。她听从了，从厨房拿起屠宰刀就去了以撒的房间。以撒一直从他房间门上的一道不小的缝隙朝外偷看，这时看到母亲拿着刀走来，开始慌张起来。他赶在母亲打开他房门之前，从他房间地毯下找到了一扇活板门跳了进去。以撒随即拿起他一直再画的画纸放到墙上，这就成为了游戏开始的封面画面。
在游戏的加载界面中，出现了以撒蜷缩成球状哭泣的形象。他的思想变成了可见的形态，有关于他母亲对他的摒弃，有关于他同龄人对他的羞辱，还有关于他自己死亡的情景。本游戏一共有13种结局，每完成一个大boss开启一个。
以撒全系列的剧情其实就是这些了，大概说明了为什么妈妈是个boss……之后的重生、胎衣、羔羊都没有剧情上的添加。