22. The Power of the Daleks 4.9-4.14
This six-part recon demonstrates the aftermath of the Doctor's first ever regeneration, where the Doctor messes with the heads of companions Ben and Polly leaving them as unsure as original viewers must have been as to whether this man was in fact the Doctor. Even if you absolute hate attempting to watch reconstructs, I recommend the first part of this one for this reaction segment alone. The division between Polly's steadfast faith in the Doctor no matter what his face, and Ben's scepticism, even downright contempt for having lost "the real Doctor" set a precedent for both possible reactions in future companions. This serial is also a chance to see this Doctor to take on the Daleks, even if it is a bit dull of an encounter, and a chance to hear them specifically called giant pepper pots, which amused me greatly.
21. Human Nature/Family of Blood 3.8-3.9
This two part adventure is important for introducing the concept of the Chameleon Arch, a watch like device which can capture Time Lord energy and render thereby its user not only non Time Lord but also alter DNA and biological structure, to make the Doctor effectively human. It has complicating side effect of alternating his personality much like regeneration does, but unlike most regenerations also completely robs him of his memories. It is important as a story arch for this particular Doctor as it furthers the struggle companion Martha Jones has in her relationship with the Doctor. It also answers a long-standing question about the Doctorthough it becomes of extra-special concern to the Ninth and Tenth Doctorabout whether he envies humanity, and what he is willing to give up for a chance at ordinary grounded human life.