The Mane 6 return to the Library from a picnic outing. Twilight calls out to Spike, and goes upstairs to find him. While she's upstairs looking, she notices that Spike's bed is out of place, and that there's a floorboard that looks a little loose. She lifts it up to find an unbound book, and upon reading it, discovers that it is a script for a play, written by Spike.
She goes downstairs with the play, where one of the others shows her the note Spike left, stating that he noticed they were running low on ink and went to get more. She shows it to them, and after a minor debate (with Rainbow wanting to, Applejack wanting not to), end up reading through it. They make several comments about the content, style etc. In particular, they note the list of ponies that Spike wanted to play the key roles, and how none of them were one of the 6 (still ponies from Ponyville, however). Overall, they decide that it's very impressive, though each has their own opinion on how it could be better. At this point, Spike walks in.
He's a little miffed when he discovers they've read it without his permission, but is quickly mollified when they compliment him on how good it is. When they express the desire to perform it for Ponyville, however, he grows nervous. When questioned, he states that he's afraid ponies aren't going to like it, which the others are quick to assure him will not be the case. He is still reluctant, and Twilight notes that there seems to be some other reason he doesn't want to perform it. Instead of pressing him, however, she simply states that it's Spike's decision, but they would really enjoy working together with him to perform for everyone. At this, Spike agrees.
Production/rehearsal begins, with the Mane 6 filling the key roles, and goes great at first. However, over time little changes to the script are introduced (Rainbow and Pinkie just do them, Fluttershy and Twilight ask permission first, Rarity and Applejack are author's choice). Spike complains each time this happens, but generally relents after some coaxing, guilt-tripping or (in Rarity's case) flirting. However, eventually one of them tries to change a key detail (the ending, a love interest, heroine's background etc.). Spike vehemently refuses to do this despite any attempt to convince him otherwise. An argument occurs in which the 6 accuse Spike of being a killjoy and a bad friend.
This causes Spike to explode at them. He rants that they're the ones who are horrible friends, always leaving him behind while they go off to have fun without him. That's why he started writing the play: it was meant to be something that was his, that he could do for himself so he wouldn't feel so left out all the time (this is why he was reluctant to have it performed). He thought that, since they were performing it together, the play would finally be his chance to hang out with them as equals. Instead, Twilight and her friends have stolen it from him, and left him out yet again.
The girls try to tell him he's wrong, but he won't hear it. He storms off, burning part of the set when they try to stop him and threatening worse if they try again. Afterwards, the girls analyse their own behaviour and realise that Spike is right: they never really include him in their activities, and now they've basically kicked him out of his own play. After some guilt and self-pity, they resolve to make it up to him.
Time-skip to opening night. Spike is still brooding, as he has ever since that day, when Celestia and Luna show up. Instead of the lecture or punishment that he was expecting, Celestia asks if he will accompany them to the play, since they want to see their son's/nephew's accomplishment. Wanting to spend time with his family - and to show off - he agrees to go despite his mixed feelings.
The play begins. Spike isn't really paying attention, until Celestia notes the performance of one of the actors. He looks to see that it is the original actor he wanted to play for that role. As the night continues, Spike notes that, in addition to his original script being performed, all the actors are the ponies he originally wanted, with the Mane 6 playing only minor roles (with at least one only being a stagehand).
The play goes well, with everypony applauding at the end. After some encouragement from his mom/aunt, he goes backstage to meet the Mane 6. They officially apologise for taking over like that, and for leaving him out so often. He goes and hugs them, and forgives them, while they promise to include him more from now on.
There could be several epilogues for this. Could have another picnic where Spike is present. Could have Spike receive a letter from a professional acting troupe, who want to perform his play Equestria-wide. Or could skip to his second play, which was written for the Mane 6 to perform the key roles.