Question: What did Booth plan on doing with his single-shot Derringer?
About the article "Assassination of Abraham Lincoln", section "Day of the assassination".

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Presidents assassinated.
Abraham Lincoln.
The assassination of Abraham Lincoln took place on Good Friday, April 14, 1865, at approximately 10:15 p.m. Lincoln was shot once in the back of his head with a .44 caliber Derringer pistol by actor and Confederate sympathizer John Wilkes Booth while attending a performance of "Our American Cousin" at Ford's Theatre in Washington, D.C. with his wife, Mary Todd Lincoln, and two guests.
Major Henry Rathbone tried to stop Booth from escaping, but Booth stabbed him in the chest and slashed his arm to the bone with a dagger that he was also carrying.
Soon after being shot, Lincoln's wound was declared to be mortal.
Lincoln died the following morning at 7:22 a.m. on April 15 without regaining consciousness.
Booth was tracked down by Union soldiers and was shot and killed by Sergeant Boston Corbett on April 26, 1865.
Booth apparently believed that killing Lincoln would radically change U.S. policy toward the South. 
 Remington Rider Single Shot Pistol Remington Zig-Zag Derringer Model 95  
Remington derringers often played critical roles in the exploits of James T. West, fictional Secret Service agent, in the American television series "The Wild Wild West".
He would carry up to three derringers: one as a concealed carry backup gun to his holstered and openly carried full sized revolver.
This derringer was carried either in a vest pocket or an inside pocket of his jacket.
Another derringer was carried as a sleeve gun under his right shirt sleeve, and the third was broken into two parts with the barrel assembly hidden in the hollowed out heel of one boot and the frame hidden in the heel of the other.
A single-shot Derringer was the concealed weapon of choice for Buford "Mad Dog" Tannen in Back to the Future Part III.
He uses it to kill Dr. Emmett "Doc" Brown in a previous unseen-on-screen timeline.
However, in the timeline seen in the movie, Tannen's attempt to kill Brown is thwarted when Marty McFly throws a pie plate at the Derringer in Tannen's outstretched hand.
The gun discharges, hitting Brown's hat and knocking it off his head.
This type of gun was a staple in the movie Django Unchained throughout the entire film. 
A derringer is generally the smallest usable handgun of a given caliber.
They were frequently used by women, because they are easily concealable in a purse or as a stocking gun.
Such weapons designed specifically for women were called "muff pistols", due to their compact size enabling them to be carried in a muff.
Derringers are not repeating firearms—repeating mechanisms such as those used in semi-automatic handguns and/or revolvers would add significant bulk to the gun, defeating the purpose.
The original cartridge derringers held only a single round, usually a pinfire or rimfire .40 caliber cartridge, with the barrel pivoted sideways on the frame to allow access to the breech for reloading.
The famous Remington derringer design doubled the capacity, while maintaining the compact size, by adding a second barrel on top of the first and pivoting the barrels upwards to reload.
Each barrel then held one round, and a cam on the hammer alternated between top and bottom barrels.
The Remington derringer was in .41 Short caliber and achieved wide popularity.
The .41 Short bullet moved very slowly, at about 425 feet per second (a modern .45 ACP travels at 850 feet per second).
It could be seen in flight, but at very close range, such as at a casino or saloon card table, it could easily kill.
The Remington derringer was sold from 1866 to 1935.
The classic Remington design remained popular even with the advent of smaller, higher-powered cartridges, made possible by the use of smokeless powder, rather than the black powder used in the 19th century and before.
A Remington-pattern derringer in .38 Special is still smaller than the most compact .25 ACP semiautomatic, and provides superior terminal ballistic performance to the .25 ACP.
While the classic Remington design is a single action, manufacturers have also made double action derringers, including some four-shot models, with the barrels stacked in a 2 x 2 block.
The COP 357 Derringer provided four shots of .357 Magnum in a package not much larger than a .25 ACP automatic, and was significantly more compact than a similar revolver.
The COP derringer was invented by Robert Hillberg and closely resembled his earlier work on insurgency weapons.
A related design, often grouped with derringers since it fits no other standard classification, is the Semmerling pistol.
It is a five-shot, .45 ACP pistol with a manual repeater: the barrel mechanism is manually pulled forward to eject the fired round, then pushed back to chamber the next round.
These pistols were originally built for the United States Army, and the few available on the civilian market are highly sought after due to their unique combination of high power, large capacity, and tiny size.
Another military pistol that is truly a derringer design is the FP-45 Liberator, a .45 ACP insurgency weapon dropped behind Axis lines in World War II.
The Remington derringer design is still in manufacture as of 2013.
Bond Arms, Cobra Arms and American Derringer all manufacture the over/under derringer in a variety of calibers from .22 long rifle to .45 Long Colt.
The current production of derringers are used by Cowboy Action Shooting reenactors as well as a concealed-carry weapon.
It is the smallest handgun that is capable of handling the largest ammunition.
Some favor the derringer as a concealed carry weapon because of its size as well as the swiftness of putting it into action.
Critics believe it is not an adequate weapon for self-defense since the derringer possesses a two-shot capacity. 
Lincoln's day started well for the first time in a while.
Hugh McCulloch, the new Secretary of the Treasury, remarked that on that morning, "I never saw Mr. Lincoln so cheerful and happy".
No one could miss the difference.
For months, the President had looked pale and haggard.
Lincoln himself told people how happy he was.
This caused First Lady Mary Todd Lincoln some concern, as she believed that saying such things out loud was bad luck.
Lincoln paid her no heed.
At around noon, while visiting Ford's Theatre to pick up his mail (Booth had a permanent mailbox there), Booth learned from the brother of John Ford, the owner, that the President and General Grant would be attending the theatre to see "Our American Cousin" that night.
Booth determined that this was the perfect opportunity for him to do something "decisive".
He knew the theater's layout, having performed there several times, as recently as the previous month.
That same afternoon, Booth went to Mary Surratt's boarding house in Washington, D.C. and asked her to deliver a package to her tavern in Surrattsville, Maryland.
He also requested Surratt to tell her tenant who resided there to have the guns and ammunition that Booth had previously stored at the tavern ready to be picked up later that evening.
She complied with Booth's requests and made the trip, along with Louis J. Weichmann, her boarder and son's friend.
This exchange, and her compliance in it, would lead directly to Surratt's execution three months later.
At seven o'clock that evening, John Wilkes Booth met for a final time with all his fellow conspirators.
Booth assigned Lewis Powell to kill Secretary of State William H. Seward at his home, George Atzerodt to kill Vice President Andrew Johnson at his residence, the Kirkwood Hotel, and David E. Herold to guide Powell to the Seward house and then out of Washington to rendezvous with Booth in Maryland.
Booth planned to shoot Lincoln with his single-shot Derringer and then stab Grant with a knife at Ford's Theatre.
They were all to strike simultaneously shortly after ten o'clock that night.
Atzerodt wanted nothing to do with it, saying he had only signed up for a kidnapping, not a killing.
Booth told him he was in too far to back out.