2x11 - War of the Worlds

The Eternal instructs Malzor on how he can go back in time to 1953 in the aftermath of the failed invasion to ensure victory by inoculating his brethren from Earth's bacteria in order to change the course of history in the favour of the Morthren race. However, Blackwood and Kincaid have followed him through the same time tunnel to stop him from succeeding in giving the aliens a huge advantage in their conquest.