The plot is complicated, so I won't go too much into here. But basically, after an event called M-Day, most of the worlds mutants were lost. For almost a year no new mutants were born and the mutant race was dying off. Then, during the also great Messiah Complex, a new mutant was born. This new mutant brought hope that the mutant race would not die off. To protect her, Cable took her off into the future.
Second Coming picks up with the sential hybrid Bastian making an attempt to not only kill off the remaining mutants, but to kill Hope(the mutant Messiah). The writing is dark and fast paced, yet still deep. There is a feeling of urgency throughout the whole book as the X-Men try to get to Hope before Bastian does. Also, a major character is killed off.
Not the major character
If you've been out of X-Men for awhile, I would recommend you getting the Messiah Complex and Messiah War graphic novels first. But with the three of them you have one of the greatest comic epics ever written. THere is also some amazing art within, especially the X-Force issues.