Upon use, deals 9,999,999 damage to every enemy in the room, then kills Isaac 3 seconds later.
Plan C does not kill both phases of bosses like The Lamb, Hush, Ultra Greed, and Mega Satan, and is most useful when used during the last phase of these multi-phase bosses.
For the game's sake, a run is not truly over until Isaac's Last Will appears. This means that some bosses that might seem to finish their death animations after Isaac dies, such as Ultra Greed, will still be technically killed and will still give their completion marks, even if Isaac does not enter the Final Boss Chest.
Some bosses have special interactions with this item:
Attempting to use this item to kill Delirium will not work. Its death animation is much longer than the time between using Plan C and Isaac's Last Will appearing, meaning Isaac will die first and it will not count as a kill. Having extra lives that revive Isaac outside the room will not help, either; Delirium must be fought again.
However, items that revive Isaac in the same room he died in, namely Lazarus' Rags (or being Lazarus), Inner Child, Soul of Lazarus, and Tainted Lost's Birthright, will allow Isaac to bypass this and kill Delirium with this item. In the case of Lazarus' Rags, though, the completion mark will go toward Lazarus and not the character originally selected.
It is possible to kill Delirium by carefully timing pauses after each screen static. Each screen static plays a cutscene that continues to play even while paused, so by pausing and unpausing at the right timings, Delirium's death animation can be shortened to just under 3 seconds allowing for his completion mark to be awarded.
Killing Mother's second phase with Plan C will kill Isaac just at the end of her death animation. Without an extra life no completion mark is awarded, but any extra life, including ones that spawn Isaac outside the room, will grant just enough extra time to receive the completion mark.
Killing Dogma's second phase with Plan C will kill Isaac while the screen is faded to black. Any extra life item, including those that revive him outside the room, will work and still take him to the Beast fight.
The Beast and the Ultra Harbingers can all be killed simultaneously by Plan C. Isaac technically dies a few frames before the ending plays, but it is still counted as a win and thus Plan C can still be safely used, even without an extra life.
Invulnerable enemies and closed Hosts will not be killed by Plan C.
Isaac's last will shows his last damage source as the cause of death, not Plan C. If Isaac has not taken damage or lost health from any source, Isaac's face is shown.
Invincibility or the Holy Mantle shield will not save Isaac from being killed.
Some items that activate when Isaac reaches zero hearts, such as Heartbreak and Spirit Shackles, will not activate and save Isaac.
Even if Isaac enters a trapdoor immediately after using Plan C, he will still die after arriving at the next floor.
Plan C has an 80% decreased item pool weight, making it less likely to appear than other collectibles.
Plan C is found in 5 item pools, the highest count amongst all items. Its lowered weight, however, offsets its presence.
Book of Virtues: Spawns 8 red wisps on the middle ring. These wisps deal 12 damage per shot. A respawn item is required to make use of them.
This is a reference to the "Morning After" pill that prevents pregnancy, commonly referred to as "Plan B". It's also a reference to a "cyanide pill", which is fatally poisonous and can be deliberately taken to commit suicide; the item's letter is coincidentally the initial for cyanide. It's likely that the item is called "Plan C" because it is too late for a "Plan B" to prevent Isaac from being born; so the "next best thing", Plan C, is for Isaac to kill himself.