Tasha's youth
Starfleet
Enterprise
Fate
Yesterday's Enterprise
Tasha's double death
Sela
In this world a young girl grows up. Her name is Tasha. Her mother, a drug addict, leaves her only when she's five. An old woman picks her up from the streets and teaches her to steal for her, and how to survive among the ruins. When Tasha's twelve, the woman dies and Tasha is alone. During the years that follow, she learns how to defend herself when there's no escape left ; gangs rape every girl they can find.
On the age of fifteen, she finally gets caught by a gang, but she's saved at the very last by a Starfleet team that beamed down on this recently discovered planet. The leader of the team is Darryl "Dare" Adin, and he takes her away from the planet, to Earth.
When they ask for her name, she chooses as surname the name of the old woman, Yar, as a rememberance to this planet to where she'll never return, but that learned her to fight and survive.

Tasha passes the finals of Starfleet Academy cum laude, and her training mission aboard the USS Starbound (under the command of captain Jarvis, and with Dare as chief of security) is also a success. That's why the Starbound, on his way back, loads a secret cargo of dilithium cristals from Starbase 36 for the Starfleet shipyards of Mars. But the Starbound is attacked by Orionians and the dilithium is stolen. 21 casualties, including all officers, except for Dare, are to be regretted.
Although it's Dare who can motivate the cadets enough to reach Starbase 18 with the crippled ship, he's arrested and convicted for treason. Dare and Yar separate as enemies, and some time later Dare escapes and disappeares.

On Treva they meet Dare, who's now working as a mercenary for the rebels, who are in reality trying to liberate the planet from the tiranny of the "legal" ruler. Data and Yar get involved in the struggle, that is about to be lost by the rebels. But the Enterprise responds to an emergency message and arrives just on time to end the fight in favour of the rebellion.
Tasha finds herself now forced to stay loyal to Starfleet and arrest Dare, although he is still (or again) her one true love. So Dare is locked up again.
But Data, a special friend from Tasha (see also Data), succeeds in proving that the evidence that was used against Dare at his trial was falsified by the Orionians, who wanted to get back at Dare for something.
Dare is re-offered his old function at Starfleet, but decides to return to his mercenaries. Tasha and Dare separate again, but this time with the promise and the hope to come back together in the future and to stay together.
Tasha's life seems better than ever...

Tasha Yar never finds the happiness that she has chased for so long. She dies while trying to save a friend.

And suddenly, Tasha is still alive and kicking.
The crew of the Enterprise D does not notice that anything changed, for them this war has always been going on. Only Guinan senses something's wrong, about things in general and about Tasha. Guinan alerts captain Picard, who chooses to believe Guinan's instincts and asks the Enterprise C to go back, attack the Romulans, be destroyed in the process, but stop this war from ever starting.
In the mean time, Tasha has started to fall in love with Lt Castillo, the helmsman of the Enterprise C, and she has found out from Guinan that, in what Guinan feels is "normal time", she has died a senseless death. When captain Garrett from the Enterprise from the past gets killed in a Klingon raid, Tasha requests to be transferred to the ship and go with it to the past and give it more chances against the Romulans.
Captain Picard agrees, and Tasha disappears with the Enterprise C through the temporal anomaly to the past. The Enterprise C attacks the Romulan vessels and is destroyed, as is the Klingon ship, but the Federation has proven its loyalty to the Klingon empire and the war is prevented. History returns to its normal course, and according to this history, Enterprises C and D never met.

But Tasha is captured during the battle and brought to Romulus, while somewhere else she's still running around as a child. On Romulus she is forced to marry a Romulan general and gets a daughter, Sela. At an attempt to escape, the child betrays her and Tasha Yar is executed. She dies twice in the same timeline...

25 years after Tasha's emprisonment, her daughter is the leader of a Romulan conspiracy that provides material support to the Klingon faction lead by the sisters (and son) of Duras, in a Klingon civil war for the leadership over the Council. If the Duras family will gain the throne and beat the Federation-loyal Gowron, there's sure to be a war between the Federation and the Klingons. That's why the Romulans assist the Duras faction. Captain Picard exposes the treason and cuts off the support. Gowron wins. Peace is safe, again.
During the conflict, Enterprise D comes into contact with Sela, who's commanding a Romulan warbird. Thus the crew of the Enterprise finds out, via Guinan, that Tasha was aboard the Enterprise C 25 years earlier, and what else happened to her later.