Another tour de force of time travel stories on Doctor Who. This episode asks the question -- How would your life be different if your decisions were different?
Clearly where we end up in life is directly determined by the decisions we make along the way. It's also determined by what goes on around us, the decisions everybody else makes, and for that matter the context we all live in on this planet in this star system in this galaxy etc. But, really, what path would life take if there were a different set of decisions?
In the episode they suggest that for most people it wouldn't make much difference. Choose one job versus another job, what difference does it make? But in the case of Donna Noble, the Doctors companion, that choice ends up being between accompanying the Doctor and not doing so. Then because the Doctor truly dies because Donna is not with him, without the presence of the Doctor the whole universe dies, step by step.