Do not delay getting advice from a local employment attorney. You can find one here on Avvo. Missouri is an at-will employment state, so unless you have a written employment agreement or are a member of a union with a collective bargaining agreement, they can let you go for any reason or no reason. Failure of a drug test is a "just cause" reason for termination provided you knew the company had a policy of drug testing and you knew that you could be fired for failing it. If this is a public employer, you may have a right to review or challenge the drug test results. If it is a private company you will likely not have the opportunity, short of a lawsuit, to challenge or review the procedures for obtaining the drug test results.
If your employer is offering you a plan of improvement in lieu of termination you should take the plan. If you refuse, you will almost certainly be fired and you will almost certainly not get unemployment benefits. Your best option will to be taking the plan of improvement and giving your best effort to be successful. Again, consult with a local employment attorney for specific answers to your legal questions.
Best of Luck!
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