As one who considers "Enterprise" an unfairly underrated series, I feel compelled to comment on segments of the sub-par fourth season such as this two-episode arc, that was, in effect, created for the sole purpose of explaining why Klingons in the original series didn't have the distinct "cranial ridges" featured in Klingons in ENT, TNG, DS9 and VOY.
Even though this "mini-arc" is an exciting and well-crafted tale, the rationale behind it is part of the reason that "Enterprise" failed as a series. Using the series to "set-up" the other Star Trek series that took place in the "future" was a sad waste of a good cast, excellent directors, and talented writers. It ultimately does a disservice to them all to use this series as little more than a device to explain events and distinctions found in the series that follow chronologically, even though they were produced years ago.
Like the much maligned series finale of "Enterprise" that employed members of TNG's cast, thus cheating the audience and the cast of a chance to make their finale about...well, about THEM, this arc is an artistic slap in the face of the Enterprise cast, writers and crew, as it wastes two episodes that could have been used to explore the relationships and develop the characters of Enterprise, instead of just "explaining" a matter of extremely minimal concern, i.e., why the Klingons in Kirk's era had smooth foreheads.
(I guess the producers felt it would have been too much to just trust that the fans of the series would understand that the reason that TOS's Klingons lacked cranial ridges was a simple matter of less developed make-up skills, and not a viral infection from a genetic augmentation experiment gone awry.)
Nevertheless, I gave it a seven (7) for some great action and good writing. The transfer of Trip by tether from the Columbia to the Enterprise at warp speed, alone, makes the two-parter worth watching!