The Earthling is dressed and acts like a typical inhabitant of planet Earth.
The four aliens: If there's some fancy way for them to enter – in puff of smoke, from above, from behind a pillar, tree etc. then do it. Otherwise, just have them enter stage right. If you can get alien costumes or weird-looking space-suits, good. Otherwise something as simple as a pair of deely-boppers and/or tinfoil will do. And if they all did different funny voices or accents, that would be good too. If the alien cop has a official-looking hat, a megaphone and/or something that looks vaguely like a weapon that would be good too. A super-soaker or baseball bat would be a cheap way of lending the right air of authority.
All roles are gender-neutral.
Scene: somewhere on planet Earth. Exterior, day. Modern times.
The Earthling is hanging about.
Suddenly the Humidorean alien enters.
Earthling: What on Earth?
Humidorean: (pacing) We Humidoreans come in peace. We have much to discuss. We must forge an alliance to defeat the Zarkwad menace! With great sacrifice we can prevail!!
Enter Zarkwad alien from another direction. Sees only Earthling.
Zarkwad: (to Earthling) Greetings. The vile Humidoreans will soon be upon you! Join with us to defeat them!!
Earthling (gestures at Humidorean) You mean him?
Humidorean: Quick. We must not-
Enter Alien cop
Alien cop: Lawbreakers! You are under arrest for interfering with an isolated planet under false pretences. Raise your appendages and cease all activity. You will not be harmed if you comply.
Enter Alien Dee. Dee sees Earthling.
Dee: I come from a distant star to offer your proud people the riches of the galaxy. Wonders undreamed of in ... (trails off since Earthling is looking far less impressed than expected. Looks around. beat.) Uh-oh.
Exeunt Zarkwad, Humidorean, Dee left, pursued by the alien cop.
Earthling's phone rings. He/she takes it out and begins to walk while speaking to it.
Earthling: Hi honey. (beat) Oh, you know. Some days are just one damn thing after another. How was yours? (exits right while continuing conversation e.g. uh-huh, yes.)
Roles are a male and female, though you could perhaps do it as two persons of the same gender if you tried. They are both seated, initially not looking directly at each other.
He: (exhales. It might be a sigh. Or it might not)
She: (looking at him) What?
She: (repeating herself) What?
He: Why what?
She: Never mind.
He: Never mind what?
She: It doesn't matter.
He: Yeah, it does. What are you on about?
She: Why did you sigh like that?
He: I didn't. I was breathing.
She: Something's on your mind.
He: No, Nothing. I was just exhaling.
She: Fine. When you exhale like that, something's bothering you.
He: It's nothing.
She: What's nothing?
He: Just leave it.
She: Something is upsetting you.
He: Are we ready to go now?
She: (exhales. It might be a sigh.) I suppose so.
(He get up and takes a step. She hasn't moved.)
With apologies to La petite mort.