Complex sentence: The event convinces the Doctor to allow his regeneration to occur as he returns to the Snowcap Station while collapsing as he regenerates into the Second Doctor moments before his companions Ben and Polly enter the TARDIS.
Split into simpler sentences: The event convinces the Doctor to allow his regeneration to occur as he returns to the Snowcap Station while collapsing. The unconscious Doctor quickly regenerates into the Second Doctor moments before his companions Ben and Polly enter the TARDIS.