"What are you wearing?" Helen asked the man at the tree-shaded bus stop, hesitating to sit down next to him on the bench. "What" wasn't the right question. She could see what he was wearing: swim goggles, a football jersey, Crocs, a kilt, a gray hoodie that was too tight on him, knee-length rainbow-striped socks, and a leather cuff around his neck with metal spikes coming out of it. Helen knew at least one person who'd have worn each item in the outfit, but would expect any pair of them to fight to the death if they were ever stuck in a room together.

The man looked down at himself, which was an effective enough way to see everything except the goggles suctioned to his forehead. He was bald, without even eyebrows, but looked too thickset and robust to have just survived cancer. Maybe he was a mental patient, picked out all his own hair... no, there was some on his arms. Surely a mental patient who picked at hair would've gotten that. Or not, Helen didn't know. "A MacGregor plaid kilt," he said, "a pair of yellow Crocs, a -"

"Never mind," Helen said.

"Do you know when the next bus that goes to the stop on Ninth Street will be?" he asked her, after a silence.

"Twenty minutes," she said, after a glance at her watch. "That's where I'm going, too."

"Really?" he asked. "Are you from that neighborhood? Do you know where Roger Swansea lives?"

Helen tilted her head. "Why are you looking for him?"

The man peered at her, assessment in his eyes. Helen shifted uncomfortably and moved one of her braids behind her ear; plastic ties clicked against each other. She didn't mind when people from her high school checked her out; older men she did mind.

"I suppose it doesn't much matter if I tell you," the man said finally. "I'd have seen the police report if you were going to call - well - anyway. Swansea's got to die," he said.

"Has he," said Helen. She kept her hands on her knees, but shifted her hips so her phone was pressed between her leg and the bench. It was there if she needed it.

"Well, you're not going to believe me," laughed the man, "but, you see, I'm a time traveler. And Roger Swansea invented a time machine. Not the same kind I used - I'm not stupid, I checked carefully for paradoxes - but today he's going to go forward in time, and he's going to bring forward a disease that they've eradicated and lost resistance to. Hundreds of people are going to die before they can stop it."

"So you decided to kill him," Helen said. "Why didn't you kill him - oh - last year? Since you're a time traveler. Why do it now?"

"Paradox checker didn't like it," the man said. "It said I could go back today - but it made me land in the bathroom of a diner outside town, was as close as I could get to his house by machine. I'm having to bus across to his place. Lucky I was able to print some currency and some clothes from this time."

"Lucky," agreed Helen absently. "But why do you have to kill the guy, not just convince him to skip his trip or go in a biohazard suit?"

"Because," the time traveler said, wagging a finger authoritatively, "history shows that he disappears on this day. If I just convince him to stay, he'll still be around - paradox in the lightcone. If I convince him to go in a biohazard suit... Well, that could actually work. Does he have a biohazard suit?"

"Not as far as I know," Helen said.

"There you go, it could take him more than a day to get ahold of one, that's probably why the paradox checker didn't say I could do that. It said I could try to kill him just fine, though."

"Won't you create some kind of paradox in the future he's going to bring the disease to?" Helen asked. "They're in your own past, if I understand right."

"Not quite," said the time traveler. "That is to say, Swansea technically landed outside my light cone - they lived on Europa, I'm from out on Argo. The only reason I got the news was via more time travel, and that means I can mess with the events that led to me getting it. It doesn't count if time travel was the only reason it could causally affect you."

"Uh-huh," said Helen skeptically.

"How long until the bus gets here?" the time traveler asked.

"Six minutes," she said, glancing at her wrist. "So you're just going to kill the man. You know he's got a family?"

"I'm going to save hundreds of lives," said the time traveler.

"In a manner of speaking," said Helen. She reached into the inside pocket of her coat, pulled out her miniature laser gun, and shot the time traveler between the eyes. He fell off the bench, the look of pious smugness still on his face.

Helen dragged the absurdly-clad body into the trees and took the long way home, rather than let the bus driver get a look at her to be questioned when the time traveler was found. Assuming he wouldn't just evaporate, or something. She didn't know how his sort of time travel worked.

When she'd finally walked the mile and a half, Helen knocked on the door to the basement. "Dad," she called. "Da-a-ad."

"I'm busy, Helen!" he shouted up the stairs.

"It's really important!"

"More important than the mess with the matter agitator?"

"I had to shoot a guy again, so about that important," she said.

Her father came halfway up the stairs. "What, again? Was he going to steal my newest invention too?"

Helen shook her head. "He was going to kill you."

Her father blinked. "Oh. Well then. Thank you, dear. What was he going to kill me for?"

"Apparently you're going to the future, on Europa?" Helen said, gesturing vaguely. "You're going to give some people a disease? Lots of them will die? The guy wanted to save them."

"Oh, I see. Well, I won't travel without adequate quarantine, then. And... I suppose if they don't die, then in the future the same person might well be born... mightn't he? Or he'll be prevented altogether, but either way he's unlikely to return to the past and try to kill me, so there is a sense in which you didn't truly... kill... someone who exists... but... How have we not been obliterated by a paradox? Dear, do you know? I was hoping to finish my machine today but if I need to spend all afternoon on math..."

Helen shrugged. "Apparently," she said, "it's safe if you get the information via time travel."

"I see. Will I need to brainwash a new therapist for you?" he asked, brow furrowing with concern.

"I think I'm okay," she said. "Easier the second time. I kind of wish you'd stop attracting assassins, though, Dad."

"You don't really need to take it upon yourself to protect me, Helen dear," he said, smiling indulgently. "But thank you."

"You're welcome, Dad," Helen said. "Love you." He took that as a dismissal and turned to go back into the basement, muttering about coefficients. Helen lugged her backpack upstairs and started her homework.