 In the second season finale, he helps Walter and Olivia to evade capture in the parallel universe, and then assists Walter in preparing equipment for them to return to the prime universe; William and Walter reestablish their friendship when William reminds Walter why he had removed the brain pieces.When Olivia returns with Peter, Bell is unable to cross over again, as his atoms would split apart and no trace of him would remain in either universe.Because of this, he sacrifices himself to provide the means for Walter, Peter, and Olivia (in actuality, the alternate Olivia) to return to the prime universe.