"Rose, why don't you tell us what we're dealing with here?" The team had been investigating the case for a week now and were still struggling to come up with an explanation. When they got to the location of the first rift activity all they'd found was a woman lying the pavement with a slightly blue face, which Owen later confirmed as suffocation. What they couldn't find out was how she had managed to suffocate. According to Owen there was no sign of smothering and even more bizarrely it seemed that all of the oxygen had left her blood not just her lungs. "So far we've got a string of bodies with seemingly nothing in common and bizarre rift activity."

"I was looking at the signals again and the travelling across parallel universes does seem to explain the rift activity." Tosh informed the rest of the team.

"Right. Rose?" Jack took a seat at the end of the table facing Rose who was still standing at the other.

"Trixams. Hostile, although you've probably figured that out by now." When Gwen showed her the pictures of all four victims she hesitated. Looking up she saw that all of the team had grim expressions mixed with determination to stop who was causing the deaths of innocent humans. "From what we can tell this is the second attack they've made on Earth, sorry, our Earth and three bodies were found in London both times, with rumours of others being found in other cities. They do take the oxygen out of the blood as well as the lungs and the reason you can't find out how is because they do it by what effectively is kissing."

"Kissing?!" Gwen asked, although it was clear they were all thinking it.

"Well not really. What they're actually doing is sucking out the oxygen from their bodies, they seem to need it to survive and can only use oxygen that has come into contact with something in the blood. To any passer by though, it just looks like a couple making out."

"A kiss that literally takes your breath away." Ianto said, but before anyone could make any other comments Toshiko had turned the conversation to more important matters.

"You said it looks like a couple to an ordinary passer by. Are you saying these Trixams look like humans?"

"No. What they actually look like is a somewhere between a Slitheen and an Ood."

"Not pretty." Jack commented.

"No, but it could be worse. There was this one I saw with the Doctor once –"

"Before we get into the attractiveness of aliens, which is a slightly weird thought anyway, do you think we could get back to the matter at hand hmm?" Owen said frowning. "How is it that they can look human?"

"We think it has something to do with taking the oxygen from the blood. Whether they take something else from it, we don't know, but it seems that once they've killed someone they can morph into them."

"How do you know all of this?"

"We managed to capture one before it came into this universe and without being able to get the oxygen it needed, it morphed back into its original form. Unfortunately it died not long after, which is what leads us to believe that they need something from the oxygen to survive."

"How many came through the rift then?" Tosh asked.

"That we know of, two. We're still unsure why they chose to come to this universe and why there are so few of them. It doesn't seem enough for a full scale invasion."

"It's not." Jack said. "They must be testers."

"What are they?" Gwen asked.

"They're sent before the majority to test out the atmosphere of a planet and find out whether they could survive on it or not. Evidently the Earth in the parallel universe was close, but not exactly what they were looking for so they thought they'd try this Earth, which judging by the second killings, it might well be."

"If it is, what does that mean for the Earth?" Tosh, along with the rest of the team were looking a little nervous.

"Take over. As they need us to survive, I assume that they'll keep some of the population to keep up the supply. However, when they first arrive, they're going to need to start getting the oxygen straight away which means we need to find those testers before they can report back to their home planet."